A nuclear medicine diagnostic apparatus includes a detector ring composed of a plurality of detectors arranged in a ring shape. The nuclear medicine diagnostic apparatus detects radiation emitted from a subject to which a radioisotope is administered by detectors in the detector ring. A typical apparatus of this kind is a PET (Positron Emission computed Tomography) apparatus. Particularly, technology development of a TOF (Time of Flight)-PET apparatus is active. The TOF-PET apparatus uses a detection time difference of a pair of annihilation gamma-rays for image reconstruction. Thus, time resolution is very important for the TOF-PET apparatus.
A detector of the TOF-PET apparatus contains a scintillator formed from a scintillator material such as LYSO, LSO, and LGSO. The length of a scintillator of a detector ring along the radial direction, that is, the thickness of a scintillator is set to exceed 20 mm. This is because the mean free path of annihilation gamma-rays within the scintillator is taken into consideration. The mean free path is an average migration length of pair annihilation gamma-rays between the time when annihilation gamma-rays enter a scintillator and the time when an interaction with the scintillator is caused (when scintillation is generated). The scintillator is set to a thickness exceeding 20 mm to obtain adequate detection efficiency of gamma rays.
However, with an increasing thickness of the scintillator, variations in interaction position of annihilation gamma-rays along the depth direction inside the scintillator increase. With increasing variations in interaction position along the depth direction, the time resolution of a TOF-PET apparatus deteriorates. With a deteriorating time resolution, image quality declines. Moreover, the scintillator is expensive. Therefore, if the thickness of the scintillator increases, the amount of scintillator used in the TOF-PET apparatus increases and manufacturing costs of the TOF-PET apparatus also rise.
Compared with a non-TOF-PET apparatus, a TOF-PET apparatus can obtain clinically sufficient image quality with a smaller number of counts so that the examination time can be reduced. Therefore, dynamic imaging is expected as a useful clinical application of the TOF-PET apparatus. However, if the number of columns of a detector ring is increased to increase the size of the field of view, the amount of scintillator increases, leading to higher manufacturing costs.